MIC28303 Datasheet, PDF (25/38 Pages) Microchip Technology – 50V, 3A Power Module

The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 5-3:
EQUATION 5-3:
V_{OUTPP} =


C^-----_O^---_U^----_T^--I--L^-----Pf^--_S^-P--_W^-----^{-----8-}
2
+
 I _{L  P P }

E
SR_COUT
²
Where:
D
C_OUT
f_SW
Duty Cycle
Output Capacitance Value
Switching Frequency
As described in Section 4.1, Theory of Operation, the
MIC28303 requires at least 20 mV peak-to-peak ripple
at the FB pin to make the g_m amplifier and the error
comparator behave properly. Also, the output voltage
ripple should be in phase with the inductor current.
Therefore, the output voltage ripple caused by the
output capacitors value should be much smaller than
the ripple caused by the output capacitor ESR. If
low-ESR capacitors, such as ceramic capacitors, are
selected as the output capacitors, a ripple injection
method should be applied to provide enough feedback
voltage ripple. Please refer to Section 5.6, Ripple
Injection for more details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON.
The output capacitor RMS current is calculated in
Equation 5-4:
EQUATION 5-4:
I_{COUTRMS}
=
_----I_{--L-----P---P----}
12
The power dissipated in the output capacitor is:
EQUATION 5-5:
P_{DISSCOUT}
=
I²
C_{OUTRMS}

ESR_COUT
5.4 Input Capacitor Selection
The input capacitor for the power stage input PV_IN
should be selected for ripple current rating and voltage
rating. Tantalum input capacitors may fail when
subjected to high inrush currents, caused by turning the
input supply on. A tantalum input capacitor’s voltage
rating should be at least two times the maximum input
voltage to maximize reliability. Aluminum electrolytic,
OS-CON, and multilayer polymer film capacitors can
handle the higher inrush currents without voltage
MIC28303
de-rating. The input voltage ripple will primarily depend
on the input capacitor’s ESR. The peak input current is
equal to the peak inductor current, so:
EQUATION 5-6:
V_IN = I_{Lpk}  ESR_CIN
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
EQUATION 5-7:
I_{CINRMS}  I_{OUTMAX}  D  1 – D
The power dissipated in the input capacitor is:
EQUATION 5-8:
P_{DISSCIN}
=
I_C
I
N
R
M
S
2


ESR_CIN
The general rule is to pick the capacitor with a ripple
current rating equal to or greater than the calculated
worst (V_{IN_MAX}) case RMS capacitor current. Its
voltage rating should be 20% to 50% higher than the
maximum input voltage. Typically the input ripple (d_V)
needs to be kept down to less than ±10% of input
voltage. The ESR also increases the input ripple.
Equation 5-9 should be used to calculate the input
capacitor. Also it is recommended to keep some margin
on the calculated value:
EQUATION 5-9:
Where:
C_IN  -^I--^O---^U----^T---^--^M_f--_S--^A-_W---^X--_-^---^-_d---_V-^--1----–-----^D-----^
dV
Input Ripple
f_SW
Switching Frequency
 2016 Microchip Technology Inc.
DS20005464A-page 25